Recently, a method has been proposed of using a catalyst comprising a metallocene compound and an aluminoxane for polymerizing olefins in the presence of the catalyst to produce olefin polymers (Japanese Patent Application Laid-Open (JP-A) Sho-58-019309, Hei-2-167307, etc.). It is known that the polymerizing method of using such a catalyst is better than a method of using a conventional Ziegler-Natta catalyst that comprises a titanium or vanadium compound and an organic aluminium compound since the polymerization activity per the transition metal in the former is extremely high and since the former produces polymers having a narrow molecular weight distribution.
Another method has been proposed for polymerizing olefins in the presence of a catalyst that comprises catalytic components of a transition metal compound and an aluminoxane or organic aluminium compound as carried on an inorganic oxide such as silica, alumina or the like (JP-A Sho-61-108610, Hei-1-101303, etc.). However, in order that the methods noted above could have a satisfactory degree of polymerization activity, a large amount of an aluminoxane must be used therein. Therefore, in those methods, the activity per aluminium used is low, and the methods are uneconomical. In addition, since a large amount of aluminium remains in the polymers formed, the methods are further problematic in that the catalyst residue must be removed from the polymers formed therein.
Still another method has been proposed of using clay minerals as catalytic components (JP-A Hei-5-25214, Hei-5-301917, Hei-7-33814). In this method, however, it is said that the clay minerals to be used must be pre-treated with organic aluminium compounds, especially with methylaluminoxane or trimethylaluminium which is extremely expensive and dangerous. In addition, the method is problematic in that the catalyst activity per aluminium therein is not satisfactory and the amount of the catalyst residue that may remain in the products is large.
In particular, the additional problem with the method is that, when styrenic polymers are produced according to the method, the catalyst activity per aluminium is much lower than that in producing other olefins according to it.
The present invention has been made in consideration of the problems noted above, and its object is to provide an efficient and inexpensive method for producing olefinic polymers and also styrenic polymers having a stereospecifically-controlled syndiotactic structure. The method is characterized by using a silane compound, and does not require a large amount of methylaluminoxane or trimethylaluminium which has poor storage stability and is dangerous and which is therefore not easy to handle. In addition, in the method, since the amount of the organic aluminium compound to be used in the total polymerization system can be greatly reduced, the metal component that may remain in the polymers produced is much reduced. Therefore, the polymers produced in the method do not require post-treatment.